Fatigue during an ultramarathon

Before we can really discuss fatigue, we first need to have a basic definition to work from. Fatigue is generally defined as a temporary decline in the ability to produce force with the skeletal muscles. This can occur from either a disruption of the working muscles, or a decline in the ability to activate that muscle. Some of you may be wondering why I’m talking about force generation in an ultrarunning blog. It’s simple, the speed you are able to while running is dependent on your ability to generate force. In running, we see fatigue in a decline in running speed.

the reasons for fatigue vary depending on the length and type of activity. In a 5k, fatigue is usually the result of temporarily altering the environment within the muscle cell. When speed increases, higher threshold muscles (muscles which are more difficult to activate) get used which due to their untrained state will produce fatiguing byproducts. These byproducts then make it more difficult to activate the muscle and reduce force production / running speed. This could occur at any race distance if the runner did not pace themselves appropriately. During an ultramarathon, you might experience this running up a steep hill. Generally speaking, this form of fatigue is short in duration and may or may not impact your performance long term in say a 100 mile race. The disruption to the muscle cell can easily be corrected, if the pace is reduced. After running a steep uphill section, you may feel tired for a few minutes and bounce back.

Another cause of fatigue is depletion of carbohydrates stores, also known as glycogen depletion. This occurs when carbohydrate usage exceeds intake over a sufficient time period to use up the stored carbohydrate. Numerous strategies have been used to avoid glycogen depletion. The most common strategy is to carbohydrate load. The recommended way to carbohydrate load is to taper mileage 2-3 weeks prior to the race, and slightly increase carbohydrate intake. This has been shown to significantly increase carbohydrate stores. While this may extend the time to carbohydrate depletion, it will likely not be sufficient in and of itself to eliminate this aspect of fatigue from 100 mile run. In order to eliminate carbohydrate depletion as a cause of fatigue you need to focus on one of two strategies. Either increase the amount of carbohydrate you can consume during the race, or decrease the amount of carbohydrate used during the race. The first strategy means training your GI tract to tolerate large volumes of fluid carbohydrate during exercise. I generally recommend training with your planned nutrition the 2-4 weeks prior to your race. It may only take ~10 days to adjust, but it’s also beneficial to train yourself to eat to your plan. The second strategy, reducing carbohydrate usage is the subject of much debate these days. First, increasing your fitness will lower carbohydrate use by boosting fat use at submaximal paces. The second way is to go on a high fat low carb diet. The fuel you use during exercise is sensitive to the food you eat. Increasing fat intake and decreasing carb intake will make your body burn more fat. However, there is little evidence suggesting this actually improves race performance, and no evidence as to the long term health consequences of ingesting ~70% of your calories from fat. Some researchers decry high carb diets as leading to accumulation of arterial plaque, but at least there are several intermediate steps to forming plaque with a sugar. That’s enough to give me pause over whether I want a high fat approach. It should however be noted that some notable ultrarunners swear by high fat diets, notably Zach Bitter the 12 hour world record holder.

The last form of fatigue I would like to mention is somewhat speculative. We know from studies of people with chronic fatigue that the immune system can decrease the ability of the brain to activate muscle through inflammatory signalling molecules. I discussed this in my previous post on overtraining. Research has shown that during the course of an ultramarathon, the working muscles and immune cells produce significant amounts of the signalling molecules. The Millet lab has also shown that voluntary muscle activation decreases throughout a 24 hour race. While speculative, it is possible that some of the decline in pace seen throughout a 100 mile race could be due to increases in inflammatory signals. How could we combat this form of fatigue? One way would be to take an anti-inflammatory medicine such as ibuprofen or Tylenol (NSAIDs). These medications interfere with production of the inflammatory signals. The research over NSAIDs has clearly shown that a using these drugs early in the race or before to stave off inflammation is a bad idea. Probably what happens is they promote faster paces in the early part of the race which later results in more inflammation. What has not been tested is whether a low dose NSAID may be beneficial later in the race after pace has already declined. It should be noted that taking an NSAID late in the race could have dire consequences if the person were dehydrated or had reduced kidney function. Please use caution since people have gone to the hospital with kindey failure from taking NSAIDs late in the race. You should probably consult your doctor if you plan on doing this. Another option would be to use low dose alcohol or a nutritional supplement such as tumeric or resveratrol.

The last thing I’ll discuss is not actually fatigue, but muscle injury. While muscle injury is classically not thought to affect a single bout performance, the length of time of an ultramarathon is sufficient to allow inflammation from the injury and reduce the ability to activate the muscle. If your race includes a lot of elevation gain and loss, then consider adding downhill running bouts to your training as this can over time reduce the severity of the injury.